Array antennas for direction finding are used to estimate the location of a source of an electromagnetic signal. An Adcock array antenna arranges four antenna elements in an orthogonal pattern for direction finding. For example, a cross is an orthogonal pattern in which each arm of the cross may be defined by two antenna elements. Typically, the antenna elements of an array antenna for direction-finding are mounted to a metallic structure, such as a tower. The tower may reflect the electromagnetic signal before or after it reaches the array antenna causing inaccuracies in measurements associated with the direction of the source of the electromagnetic signal. The electromagnetic signal may be modeled as an electromagnetic field or as a planar wavefront incident upon the antenna. Any other metallic objects in the vicinity of the antenna may perturb the electromagnetic field; hence, degrade the accuracy of direction finding. Thus, a need exists for reducing or eliminating the influence of unwanted reflections from surrounding metallic objects on a direction-finding antenna.
A direction-finding antenna is frequently used in applications where space conservation is of paramount importance. Aesthetic appearance of a direction-finding antenna may be facilitated by disguising the antenna to meet local zoning ordinances or contractual leasing requirements of an antenna site. Even where aesthetic appearance does not, in effect, limit size of the antenna, available space at a crowded antenna site or building roof-top may limit the practical maximum antenna size. Therefore, the direction-finding antenna may need to fit a particular volume constraint or an area constraint for mounting on a building, a tower, a monopole, a vehicle, or an airplane.
Wireless communication systems are expected to incorporate location-determining equipment, such as direction-finding antennas because of emergency "911" service, public safety, and law enforcement concerns. As cellular networks operate side-by-side with personal communication service networks on different frequency bands, person communication service infrastructure costs may be reduced by sharing existing cellular sites. Yet, the full benefit of sharing such infrastructure costs may only be realized with the availability of dual-band products. Thus, a need exists for a dual-band antenna for operation within the cellular band and the newer personal communication service (PCS) bands within the U.S. In addition, similar dual-band antennas are needed for the European cellular band and the newer personal communication network (PCN) in Europe.